1. Field of the Invention
The invention relates to an improved self-boosting electromechanical friction brake for a motor vehicle.
2. Description of the Prior Art
The invention is explained below in terms of a disk brake, because known friction brakes of this generic type are typically disk brakes, and because the invention can be explained clearly in terms of a disk brake. However, the invention is not limited to this type of brake.
Known electromechanical friction brakes have a friction brake lining, which for braking can be pressed by an electromechanical actuator against a brake body. In the case of a disk brake, the brake body is a brake disk. The actuator typically has an electric motor and a rotation-to-translation conversion gear, which converts a rotary driving motion of the electric motor into a translational motion for pressing the friction brake lining against the brake body. A step-down gear, such as a planetary gear, is often interposed between the electric motor and the rotation-to-translation conversion gear. As rotation-to-translation conversion gears, helical gears are known; because there is less friction, rolling threaded drives such as roller threaded drives are preferably used. However, a rotatable cam can also be used, for instance, as the rotation-to-translation conversion gear for converting the rotating driving motion of the electric motor or of the step-down gear into a translational motion for pressing the friction brake lining against the brake body.
Electromechanical disk brakes moreover have a further friction brake lining, which is pressed against the brake body by a reaction force, which is caused by the pressing of the one friction brake lining against the brake body. For generating the reaction force, so-called floating calipers are known, in which the friction brake linings are placed on both sides of a brake disk. The floating caliper is supported displaceably transversely to the brake disk. When the one friction brake lining is pressed by the actuator against the one side of the brake disk, the floating caliper is displaced transversely to the brake disk and presses the further friction brake lining against the other side of the brake disk.
To attain a self boosting, in known electromechanical disk brakes, self-boosting devices with a wedge mechanism are used. In them, a friction brake lining is movable in the rotary or circumferential direction of the brake disk and has a wedge on a side facing away from the brake disk. Via the wedge, the friction brake lining is braced on a counterpart wedge face in the brake caliper, which forms an abutment for the wedge. If the friction brake lining is pressed for braking against the brake disk, the latter exerts a frictional force in the direction of rotation of the brake disk on the friction brake lining, which urges the latter in the direction of an increasingly narrower wedge gap between the counterpart wedge face and the brake disk. Because of the wedge principle, the wedge mechanism exerts a force on the friction brake lining that has a component that is transverse to the brake disk. This transverse component forms a contact pressure, which presses the friction brake lining against the brake disk in addition to a contact pressure exerted by the actuator. As a result, the braking force is boosted. The self-boosting device converts the frictional force, exerted by the rotating brake disk against the friction brake lining pressed against it, into the additional contact pressure. The wedge mechanism may have a wedge angle that is either constant or varying over a displacement travel of the friction brake lining, and as a result the self boosting is either constant or varies with the displacement of the friction brake lining. The wedge mechanism is a mechanical self-boosting device. As a mechanical self-boosting device, a lever mechanism is for instance also known, in which the friction brake lining, on being pressed against the brake body, is braced via an obliquely positioned lever stressed for tension or pressure and as a result brings about the self boosting. A support angle at which the lever supports the friction brake lining obliquely to the brake disk corresponds to the wedge angle of the wedge mechanism. Nonmechanical self-boosting devices are also possible, such as a hydraulic self-boosting device. Typically, the self-boosting device acts on the friction brake lining that is acted upon by the actuator, but this is not compulsory; the self-boosting device may also act on a friction brake lining not acted upon by the actuator.
A coefficient of friction between the friction brake lining and the brake body varies with operating conditions, such as a rotary speed of the brake body or in other words a relative speed between the brake body and the friction brake lining, the temperature, soiling, and wetness; at a constant actuation force of the actuator, the contact pressure exerted by the self-boosting device of the friction brake lining against the brake body, and thus a braking force or braking moment, therefore also vary to the same extent. As a consequence, at a low coefficient of friction resulting from a fluctuation in the coefficient of friction, a major actuation force and thus high actuation energy must be exerted by the actuator in order to generate a defined braking force. To overcome this problem, a self-boosting device with a wedge mechanism with an adjustable wedge angle, that is, a settable self boosting, has been proposed. The present invention discloses a different course.